In many applications that require the use of active devices, like radio frequency communication or radar detection, it is often desirable to operate the active device in a highly efficient manner over a wide output power range and a wide frequency range. In the case of high-efficiency RF power amplifiers with variable output power, this can be achieved by using a variable supply voltage/current. Variable DC power supplies are typically realized by silicon based DC/DC converters. However, this solution is expensive and typically not suited for full integration in combination with state-of-the-art power amplifiers used in wireless communication devices.
Another factor that determines the efficiency of an active device is the impedance of the load at the output of the active device. However, providing a load with the optimal impedance for a particular active device over a wide range of working conditions of the active device can be difficult to achieve.